1. Field of the Invention
The present invention relates to a road surface frictional coefficient estimating apparatus which estimates the frictional coefficient of a road surface on which a vehicle is traveling.
2. Description of the Related Art
As techniques for estimating the frictional coefficient (hereinafter referred to simply as “μ” in some cases) of a road surface on which a vehicle is traveling, the techniques disclosed in, for example, JP Patent Publication No. 3669668 (hereinafter referred to as “patent document 1”) and Japanese Patent Application Laid-Open No. 2003-118554 (hereinafter referred to as “patent document 2”) have been proposed by the present applicant.
According to the technique disclosed in patent document 1, a road surface reaction force acting on each wheel from a road surface (a cornering force (a lateral force of a vehicle) and a braking/driving force (a longitudinal force of the vehicle)) is estimated using a tire characteristic set on the basis of an estimated value of μ. Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of a yaw rate change velocity of the vehicle (the yaw rate change velocity at the center of gravity of the vehicle), which are motional state amounts of the vehicle and which occur due to the resultant force of the road surface reaction forces, are calculated. Further, according to the technique disclosed in patent document 1, a previous estimated value of μ is updated on the basis of the difference between the value of the lateral acceleration detected by an acceleration sensor and the estimated value of the lateral acceleration or the difference between the differential value of the yaw rate values detected by the yaw rate sensor (the detected value of the yaw rate change velocity) and the estimated value of the yaw rate change velocity, whichever difference is greater, thereby determining a new estimated value of μ.
According to the technique disclosed in patent document 2, a tire model set on the basis of the estimated value of μ is used to estimate the road surface reaction force acting on each wheel from a road surface (the cornering force and the braking/driving force). Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of the longitudinal acceleration of the vehicle indicative of the motional state amounts of the vehicle generated by the resultant force of the road surface reaction forces are calculated. According to the technique disclosed in patent document 2, in the case where a slip angle (side slip angle) of a rear wheel is small, the estimated value of μ is sequentially updated by incrementing or decrementing the estimated value of μ by a predetermined value according to a magnitude relationship between the estimated value of the longitudinal acceleration of the vehicle and the detected value of the longitudinal acceleration provided by the sensor. In the case where the slip angle of a rear wheel is large, the estimated value of μ is updated by incrementing or decrementing the estimated value of μ by a predetermined value according to the magnitude relationship between the estimated value of the lateral acceleration of the vehicle and the detected value of the lateral acceleration provided by the sensor.
The road surface reaction force acting on a wheel depends not only on μ but also on the slip rate or the side slip angle (slip angle) of a wheel. For this reason, according to the techniques disclosed in patent documents 1 and 2, the slip rate of a wheel is estimated and the side slip angle of a vehicle or the side slip angle of a wheel is also estimated using a motional model of the vehicle.
Meanwhile, the direction of the actual lateral acceleration or longitudinal acceleration of the vehicle or the direction of the yaw rate change velocity naturally is not always fixed to a certain direction. Hence, the estimated value of each of the lateral acceleration, the longitudinal acceleration, and the yaw rate change velocity (the estimated value of μ and the value obtained by using a model, such as a wheel friction characteristic model) or the detected value thereof (the value obtained by a sensor) will have positive or negative polarity indicative of the direction of each of the lateral acceleration, the longitudinal acceleration, and the yaw rate change velocity.
On the other hand, for example, the estimated value and the detected value of the lateral acceleration carry a meaning as the observed values obtained by observing the same object, namely, the lateral acceleration, of an actual vehicle in different ways, so that these two values should basically share the same polarity. This applies also to the relationship between the estimated value and the detected value of the longitudinal velocity and the relationship between the estimated value and detected value of a yaw rate change velocity.
Meanwhile, there are cases where, for example, if the absolute value of an actual lateral acceleration, an actual longitudinal acceleration, or an actual yaw rate change velocity of a vehicle is relatively small, the estimated value and the detected value develop polarities that are opposite to each other due to an error component included in the estimated value or the detected value. In such a case, it is highly likely that the difference between the estimated value and the detected value contains relatively many unwanted components that are not dependant upon an error of an estimated value of μ used to determine the estimated value.
According to the technique disclosed in patent document 1 described above, the polarities of the lateral acceleration and the yaw rate change velocity used for estimating μ are not taken into account. Similarly, the technique disclosed in patent document 2 described above does not consider the polarities of the lateral acceleration and the longitudinal acceleration of the vehicle used for estimating μ.
Hence, according to the aforesaid techniques disclosed in patent documents 1 and 2, in the situation wherein the estimated value and the detected value of each of the lateral acceleration, the longitudinal acceleration and the yaw rate change velocity of the vehicle have polarities that are opposite to each other due to the influence of error components contained therein, the estimated value of μ may be inconveniently subjected to improper updating based on the difference. In other words, there is an inconvenient case where the estimated value of μ is improperly updated on the basis of the difference in the situation wherein it is highly likely that the dependency of the difference between the estimated value and the detected value upon an error of an estimated value of μ is low. Inconveniently, therefore, according to the techniques disclosed in patent documents 1 and 2, the estimated value of μ develops unstable changes, i.e., the estimated value diverges, or the accuracy of the estimated value deteriorates.
Further, the difference between the estimated value and the detected value of the lateral acceleration and the difference between the estimated value and the detected value of the yaw rate change velocity are also susceptible to an estimation error or the like of a side slip motional state amount of the vehicle in addition to an error of an estimated value of μ. For this reason, even if the estimated value of μ is updated on the basis of the aforesaid difference on the lateral acceleration or the aforesaid difference on the yaw rate change velocity, the error of the estimated value of μ may not be properly reflected. In such a case, it is difficult to determine the estimated value of μ accurately and stably.
In addition, according to a finding of the present inventor, the change rate of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity relative to a change in the μ of a road surface, i.e., the sensitivity of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity in response to a change in the μ, changes according to a behavior state or the like of the vehicle. In a situation wherein the sensitivity is low (in a situation wherein the sensitivity is close to zero), the dependency of the difference between the estimated value and the detected value of each of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity of the vehicle will be low. Hence, in the situation wherein the sensitivity of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity in response to a change in the μ is low, it is considered desirable to restrain the updating of the estimated value of μ on the basis of the difference between the estimated value and the detected value of each of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity of the vehicle or on the basis of a magnitude relationship between the estimated value and the detected value.
However, the techniques disclosed in patent documents 1 and 2 mentioned above do not take the aforesaid sensitivity into account. This leads to a possibility that the estimated value of μ is undesirably updated unduly even when the sensitivity of the lateral acceleration, the longitudinal acceleration or the yaw rate change velocity in response to a change in μ is low. As a result, the estimated value of μ inconveniently exhibits unstable changes or the accuracy of the estimated value of the μ undesirably deteriorates.